Many of the organic polyisocyanates currently commercialized are manufactured in a phosgenation process. For example, TDI may be manufactured by contacting in a reaction zone toluene diamine (TDA) with an excess of phosgene (COCl.sub.2) in the presence of a solvent. The products of the reaction between TDA and COCl.sub.2 are TDI and hydrogen chloride. Most of the hydrogen chloride is a vapor removed at the overhead of the reaction column for collection and eventual sale as aqueous hydrochloric acid. Excess phosgene is also removed from the reaction column. The bottoms in the reaction column, comprised of solvent, TDI, remaining COCl.sub.2, and small amounts of HCl and byproducts flow through one or several distillation towers in which the majority of COCl.sub.2 is removed, condensed, and eventually recycled back to the reaction zone. In the distillation of COCl.sub.2, other impurities are also vaporized and caught in the COCl.sub.2 stream, such as solvent, byproducts, water, and HCl, which are desirably removed prior to recycling COCl.sub.2 to the reaction zone.
In one process, phosgene-containing vent gases, along with other impurities such as HCl, are collected from the various sources and fed into an absorption tower. This phosgene stream is washed down in the absorption tower with toluene allowing HCl and other gases to vent off at the overhead of the tower, thus removing the vast majority of HCl. The phosgene-loaded toluene stream then may enter a phosgene stripper tower where phosgene is stripped by heat from the toluene. The stripped phosgene stream is fractioned at a sidedraw or the overhead of the tower as a liquid flowing through coolers to drums for re-use back in the reaction zone. The liquid phosgene stream drawn from the stripping tower, or for that matter from any source, is termed herein as a "liquid phosgene stream" and comprises at least 60 weight percent phosgene, preferably at least 80 weight percent, more preferably at least 84 weight percent. The remainder of the liquid phosgene stream comprises one or more solvents, such as monochlorobenzene, dichlorobenzene, toluene, benzene, xylene, methylchloride, methylene chloride, chloroform, carbon tetrachloride, ethyl chloride, dichloroethane, trichloroethane, dichloropropane, esters and diesters of aliphatic and aromatic carboxylic and dicarboxylic acids having less than 20 carbon atoms, aliphatic ethers having less than 10 carbons, and trace amounts of water and HCl.
The water traces unavoidably enter the process through their presence in the toluene used for washing, through raw materials such as the solvent for phosgenation or the toluenediamine (TDA), and/or through non-airtight joints which can develop at various locations in the process, for example, in vacuum parts of the plant. It is the presence of these trace amounts of water in a recycle phosgene stream which presents various problems. Applicants have found that the traces of water in the process present at least three distinct and interrelated problems. Traces of water in the reaction zone react with TDI to form urea byproducts, thus lowering the TDI yield. Similar losses occur in other phosgenation processes. Second, the urea byproducts must be disposed of which increases chemical wastes. Third, the presence of water will render the otherwise dry HCl in various plant sections corrosive, thereby degrading the metal walls of equipment like the phosgenation reactor, heat exchangers, separation columns, which come in contact with the HCl. These trace amounts of water translate into several pounds of water per hour based on a liquid phosgene recycle rate of a few to several hundred tons per hour. Therefore, it is highly desirable to reduce the amount of water in a liquid phosgene stream prior to entry into the reaction zone.
Typically, water reacts spontaneously with phosgene forming HCl and carbon dioxide products at high water concentrations. However, the amount of water in a typical liquid phosgene recycle stream is so low-on the order of 200 ppm to 1,500 ppm--that the reaction proceeds only very slowly, if at all. It has been proposed in DE 3333720 C2 to employ a special anionic exchanger whose structural properties and manufacture are also claimed in the patent to remove water traces from a solvent recycle stream containing a predominant amount of solvent and low levels of phosgene. This reference, however, does not address the problem of water contamination in a phosgene recycle stream, predominantly comprised of phosgene, but rather identifies the solvent stream as the source of water contamination.